Hybrid vehicles and electric vehicles have recently been of great interest as environment-friendly vehicles. The hybrid vehicles are now partially commercialized.
A hybrid vehicle has, as its motive power sources, a DC power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby securing the motive power source as well. An electric vehicle refers to a vehicle that has, as its motive power sources, a DC power supply, an inverter and a motor driven by the inverter.
Regarding the hybrid or electric vehicle, it has been proposed to boost the DC voltage from the DC power supply with a voltage step-up converter and supply the boosted DC voltage to the inverter which drives the motor (Japanese Patent Laying-Open No. 2001-275367).
Specifically, the hybrid or electric vehicle has a motor drive apparatus mounted thereon that includes a bidirectional converter and an inverter. The bidirectional converter includes two IGBTs (Insulated Gate Bipolar Transistors) connected in series between a power supply line and a ground line of the inverter and a reactor L1 having one end connected to an intermediate point between the two IGBTs and the other end connected to the positive electrode of a battery.
The bidirectional converter boosts a DC voltage supplied from the battery in such a manner that an output voltage thereof is equal to a voltage command, and supplies the output voltage to the inverter. In a case where the DC voltage is to be raised to the output voltage by means of the bidirectional converter so that the output voltage is equal to the voltage command, a control unit conducts feedback control using a PI control gain. The inverter then drives the motor with the DC voltage supplied from the bidirectional converter.
As seen from the above, the conventional motor drive apparatus boosts the DC voltage from the battery to drive the motor with the boosted DC voltage.
The conventional motor drive apparatus, however, encounters a problem of overvoltage and overcurrent that could be generated when the feedback control is conducted for making the output voltage equal to the voltage command in such a case where the output voltage of the bidirectional converter varies due to a variation of a load applied to the motor.
FIG. 10 is a voltage timing chart. Referring to FIG. 10, as the load applied to the motor increases, the output voltage of the bidirectional converter decreases below the voltage command, resulting in an increase in deviation between the voltage command and the output voltage.
In such a case, if a proportional gain and an integral gain determined according to the deviation are used to carry out the feedback control, the output voltage of the bidirectional converter exceeds the level of the overvoltage to cause the problem of generation of overvoltage and overcurrent. More specifically, since the proportional gain increases according to the deviation and the integral gain increases due to integration of the deviation, the output voltage of the bidirectional converter sharply increases to exceed the overvoltage level, resulting in generation of the overvoltage and overcurrent.